What clothing the different races can equip is defined in the clothing XML. In the CorePantsSkirts.xml file, for each GameItemType node there are SupportedUnitModelType subnodes which determine which models are able to equip that piece of clothing.
Changing it by hand looks like it'll be a pain, I'll probably knock something up in C# to do it easier if you can wait a day or two.
If you remove the SupportedUnitModelType however, it's available to all. That may be an alternative, but still a pain unless you can batch script your way through cleaning up the files properly.
